by T.N. Baranger, Université de Lyon1, LaMCoS UMR5259
While we are walking the front of our shoe soles undergo a bending movement when we put our weight on our forefoot. Repeating this movement can lead to a shear which the Bennewart laboratory test can replicate.
This test is used to estimate the lifetime of a shoe sole. It produces repeated bendings so as to check if a crack appears or if it can spread from a cut after a very large number of bending cycles (100.000 cycles), that is 124-mile walk (200 km). The important local extensions and the cyclical movement undergone by the sole can generate crack risks.
In order to avoid having recourse to "real" tests on numerous sole prototypes - which would quickly become expensive - the CTC (Leather Technology Center) and the LaMCoS (Laboratory of mechanics and solid structures, UMR5259 Université de Lyon) are developing a Virtual Bennewart test.
Because the soles are often elastomeric, the numerical simulation of their behaviour often requires new developments. The mechanical behaviour of these materials are governed by constitutive laws which couple several phenomenons: hyperelasticity, viscoelasticity and damage of Mullins. This law was integrated in Code_Aster and is currently being validated. The identification of the parameters for each phenomenon is realized thanks to simple tests coupled with an image correlation system.
Salome-Meca proved to be the appropriate digital platform for this type of applications. It can import complex geometry from CAD (IGES) or from digitisation (STEP).
A study about the development of a law of fatigue is underway. Once it is implemented in Code_Aster we will be able to simulate the bending dynamic test over several cycles - i.e. the Virtual Bennewart test. The medium-term goal is not only to determine the number of cycles leading to the appearance of a cut, but also to quantify the spread of a crack on a sole subjected to repeated bending.